Combined effects of folivory and neighbor plants on Cirsium altissimum (tall thistle) rosette performance

Predicting how herbivory and neighbor plant interactions combine to affect host plants is critical to explaining variation in herbivores' impact on plant population dynamics. In a field experiment, we asked whether the combined effects of neighbor plants and folivores upon performance of tall thistle (Cirsium altissimum), a monocarpic perennial, can be predicted as the product of their individual effects (i.e., effects of neighbor plants and folivores act independently in suppressing tall thistle performance). Alternately, the combined effects of neighbor plants and folivores might be greater, indicating a synergistic interaction, or less, indicating an antagonistic interaction, than the product of their individual effects. Our experiment involved a neighbor plant clipping treatment and a folivory reduction treatment in a factorial design with manipulations applied to naturally occurring tall thistle rosettes in restored tallgrass prairie. Clipping neighbors at the soil surface within 40 cm of tall thistle rosettes increased light availability to rosettes, rosette growth, and the transition rate of 2007 rosettes to reproductive adults in 2008. Folivores' and neighbor plants' effects acted independently upon rosette growth. By contrast, folivory reduced the rate at which 2007 rosettes transitioned to reproductive adults in 2008 only where neighbor plants were unclipped, indicating a possible synergistic interaction of neighbor plants and folivores in suppressing tall thistle performance. Our results suggest that (1) promoting neighbor plant aboveground biomass should suppress rosette-forming weeds, and (2) folivory, which reduces light acquisition by rosettes, may generate synergistic herbivory x neighbor plant interaction effects on rosettes in grasslands, where light often limits rosettes.